There are two main types of hand held metal detectors in common use. A first type of metal detector is referred to as an Induction Balance detector or more commonly a very low frequency “VLF” metal detector, as they operate using very low frequencies. That type of metal detector operates in the frequency domain and uses a sinusoidal transmit signal to transmit a varying magnetic field.
A second type of metal detector called a Pulse Induction or “PI” detector operates in the time domain. The PI metal detector transmits a magnetic field due to a transmit coil being energized by high current pulses.
Most VLF metal detectors use a searchcoil assembly which contains a transmit and a receive coil and a feedback or bucking coil. The coils are physically arranged in such a way as to cause a null in the receive coil to the signal from the transmit coil. Metallic objects entering the searchcoil's field upset the balance, causing a signal in the receive coil. The received signal has a phase and amplitude which is dependent on the type of metal as well as the size and shape of the metallic object.
As the transmit signal is sinusoidal, many metal detectors incorporate the transmit coil into an oscillator circuit, with the transmit coil becoming the oscillator's tank circuit. The continuous sinusoidal signal provides a phase reference for one or more synchronous demodulators or synchronous sampling circuits which output a DC signal containing information regarding the phase and amplitude of the receive signal.
By adjusting the clock phase of the synchronous demodulators or sampling circuits relative to the sinusoidal transmit reference, it is possible to obtain a DC signal which has essentially no component due to mineral ground. The synchronous demodulators or synchronous sampling circuits output a DC signal which is, for example, positive for non ferrous targets and negative for ferrous targets.
Only positive signals are chopped at an audio frequency and amplified to a speaker or headphones to indicate the presence of a metallic object which falls within a predetermined range.
The positive or negative DC signal may also be applied to a meter.
Pulse Induction metal detectors operate by switching high current pulses through a transmit coil. The current causes a magnetic field to surround the transmit coil. The field causes eddy currents to flow on the surface of metallic objects. Either the same coil used for transmit or a separate receive coil picks up signals from the metallic targets after the transmit pulses terminate. The received signal is sampled one or more times and processed so as to provide a DC signal to cause a voltage controlled oscillator to output an audio amplitude and/or a frequency change of the audio output stage to indicate the presence of a metallic object.
A further detailed description of the above metal detector technologies is not provided, as both methods are well known in the art.
Both Induction balance—VLF and Pulse Induction “PI” metal detectors have a number of drawbacks.
One drawback with VLF metal detectors is that they are very difficult to build using a resonant tuned receive coil. Small temperature changes in the searchcoil can translate into a phase shift of the receive signal, which in turn causes errors in the ability of the metal detector to cancel signals due to mineral ground and to discriminate against unwanted metallic objects. The effect however of using a non-resonant tuned coil is a major reduction in the metal detector's sensitivity and poor interference rejection capabilities.
Another drawback of most VLF metal detectors is that their detection depth is limited due to the low power of the transmitted signal.
Pulse Induction or PI metal detectors generally have greater depth capabilities than VLF metal detectors. PI metal detectors, however, have, at best, only a very limited discrimination ability.
Minerals such as iron oxides in the ground also adversely effect a metal detector's discrimination ability often to the point where discrimination between different metallic objects is not possible.
Accordingly, there is a need for a metal detector which has the ability to work with a tuned searchcoil, has a major depth capability, superior noise rejection and the ability to discriminate against a range of unwanted metallic objects which are buried in highly mineralized soil.